1. Field of the Invention
The present invention relates to a gasket for an electrochemical cell, and to an electrochemical cell.
2. Description of the Related Art
Generally, as a power source for backup or the like in a clock function or a semiconductor memory, various electrochemical cells are used so as to realize high energy density and light weight. As a method of mounting the electrochemical cell, for example, reflow soldering is adopted in the case of an electric double layer capacitor.
In the reflow soldering, a printed wiring board on which the electrochemical cell is mounted is heated to a temperature of 200° C. or higher. Therefore, it is important to ensure heat resistance during the reflow soldering so as to improve the sealing performance, for improving reliability of the electrochemical cell.
The electrochemical cell includes a case which serves also as a positive electrode can, a cap which closes an opening part of the case and serves also as a negative electrode can, and an insulating gasket sandwiched between the case and the cap. The electrochemical cell has a structure in which an inner edge of the opening part of the case is caulked with a flange of the cap via the gasket so as to seal electrodes and an electrolyte housed inside.
Conventionally, in order to improve the sealing performance of the gasket, the maximum thickness of an upper part of an outer wall of the gasket is set to a larger value than the minimum thickness of a lower part of the outer wall, and the radius of the upper part of the outer wall of the gasket is set to a larger value than the radius of the lower part of the outer wall (see, for example, Japanese Patent Application Laid-open No. 2005-123017). Because a maximum compression part of the gasket is formed thick, the gasket is less prone to be broken. In addition, because an engaging margin between the negative electrode can and the gasket is increased, the sealing performance is improved. However, because of the increased engaging margin, it is difficult to insert the negative electrode can into the gasket that is inserted into the positive electrode can. Further, a sealing agent applied to the gasket may be removed due to friction between the gasket and the negative electrode can. As a result, it is difficult to improve the sealing performance of the electrochemical cell.
Therefore, there is disclosed a gasket equipped with a guide having an inner radius that is substantially the same as an outer radius of the flange of the negative electrode can which is fit in an annular groove of the gasket, and a housing part extending from a bottom to the outside in the radial direction for housing the sealing agent (see, for example, Japanese Patent Application Laid-open No. 2008-60158). This guide can improve the workability of insertion.
However, although the workability of insertion of the negative electrode can is improved, the engaging margin is small. If it is tried to increase the engaging margin between the gasket and the negative electrode can so as to improve the sealing performance, the sealing agent applied to the gasket is removed by the negative electrode can. On the contrary, in order to improve the sealing performance by the sealing agent, it is necessary to ensure that the sealing agent is not removed. For this purpose, the engaging margin between the gasket and the negative electrode can is required to be decreased. In this way, there is a problem that the large engaging margin cannot be compatible with keeping the sealing agent without removing the same. In addition, there is another problem that even if the workability of insertion of the negative electrode into the gasket can be improved, the sealing agent applied to the gasket may be removed due to a slight change of the insertion state such as an inclination or a displacement of the negative electrode can.